Drake 2B in SDR mode

[AB2EZ]

One of the things I have wanted to try is:

Drake 2B Q-multiplier interface (on the back of the receiver) => Soft Rock (or equivalent) 455kHz-to-11kHz mixer => sound card/computer running Power SDR.

I tried it yesterday (newly-acquired Drake 2B), and it works great!

While one could use the 50kHz i.f. output of the 2B, you would be limited by the narrow bandwidth of the 2B's 3rd i.f. output filter. The 455kHz output has plenty of bandwidth... and the 2B's 2nd i.f. transformers appear to do a good job of attenuating the local oscillator in the 2nd i.f. mixer.

I used the following interface: Q-multiplier connector (interface to the 455 kHz i.f.) on the back of the 2B => an LM386 (plain vanilla) amplifier, AC-coupled to the 2B's i.f. output with a 0.01uF capacitor => to the Soft Rock (or equivalent) input.

The LM386 presents a 50k Ohm input to the Drake 2B's i.f. output; and the LM386 provides a gain a 20 (26dB), and the ability to drive a 50 ohm coaxial cable. It has enough bandwidth to handle 455kHz.

Stu

[WD8BIL]

It's fun, Stu.

I'm now using the Drake R4A with a SoftRock Lite right off the 1st I.F. at 5645Khz. The power SDR software and a Soundblaster Live card in the 'puter.

Have fun OM!

[WD5JKO]

Stu,

   Your adaption of Soft Rock to different BA receivers is interesting. I bought a SR kit and never built it up. You inspire me to move forward with this. Your use of an LM386 at 455 Khz is interesting since the gain is falling like a rock as the frequency approaches 1 Mhz.  It reminds me once where I used a 78L05 with a 2N7000 at the common pin to ground where the 78L05 drove 75 coax cable. I built a digital video switcher where 4 cameras had their own quadrant on a video monitor. The little 3 terminal voltage regulator drove 5 mhz video BW to a monitor very well with perfect picture.

   Want to put my Soft Rock on my G76 receiver 1st IF output. Then combine that with a DDS VFO, and the G76 will be the ticket. That rig on xmit does a solid 65 watts RF out on 80-10m with plate modulation. The receiver lacks big time, and the stock VFO are not the best...
   
   I used the Gonset G76 with the Gonset GSB201 (5db pad in between) during the AM Rally, and I got 150-200w AM on all bands tried (80-15). Adding Soft Rock, and a good VFO, and then that would be the ticket. 


Jim
WD5JKO

[AB2EZ]

Jim

Yes...

Using the LM386 at 455 kHz is a stretch... but it still has plenty of gain (particularly current gain: a 50k Ohm input impedance, and an 8 Ohm output impedance). With the frequencies of interest being: 455 +/- 11kHz, the slope of the voltage gain vs frequency at 455 kHz isn't a problem. In the past, I have used a Darlington pair of 2N3904's to transform the impedance at the i.f. pick-off point... and that also works well. An FET source follower would also work.

I always learn something about the details of a receiver when I hook a Soft Rock (or equivalent) up to it. [I find the Japanese quadrature mixer kits more relaxing to build].

In the case of the Drake 2B, I decided that there were too many strong out-of-band signals (e.g the 405kHz local oscillator) at the point where I was picking up the 455 kHz i.f. output. These seemed to be causing lot's of spurious responses in the SDR application.

Fortunately, I happened to have, on hand, a Murata 455 kHz crystal filter with a 12kHz bandwidth [These cost only a few dollars].

So, what I did was to take the output of the LM386, and put it in series with a 2200 Ohm resistor and a 0.01uF capacitor to drive the Murata filter. I loaded the output of the Murata filter with a 2200 Ohm resistor; and I ran that into a second LM386 (with a resistive attenuator to keep from overloading the second LM386).

That worked great! All of the spurs went away, and it still works great with the 12kHz i.f. bandwidth.

I also monitor the i.f. output on a scope... and I noticed that the level of the i.f. output was pretty strongly dependent on the signal level coming into the 2B. This dependence wasn't big enough to cause problems for the Soft Rock or the Power SDR application ... but it was interesting.

The reason for this is (of course), that the Drake 2B has three (3) i.f. amplifiers under AGC control; and one of those i.f. amplifiers is the 3rd i.f. amplifier... which is after the 455 kHz pick-off.

That variability would not be anywhere near as large if I took the i.f. output of the Drake 2B at 50kHz... but then I would be limited by the 3.6 kHz bandwidth of the Drake 2B's 3rd i.f. filter.

If I have a reason to take the 2B out of its cabinet, I may modify the AGC chain so that the grid bias on the 3rd i.f. tube is fixed... rather than being controlled by the AGC. That will reduce the AGC range somewhat, but it will stabilize the 455 kHz i.f. output.

Best regards
Stu

[AB2EZ]

Attached is the schematic of the interface.

It should work with almost any 455 kHz i.f. (attenuated, if necessary; and taking the B+ at the pick off point into account when selecting the voltage rating of the coupling capacitor)

With this approach, you should be able to pick off the 455kHz i.f. signal before the 455kHz i.f. filter... and, therefore, you should be able to set the i.f. bandwidth presented to the Soft Rock (or equivalent) with the choice of the MuRata filter.

I'm currently using a filter with a 455kHz +/- 6kHz (6dB) specification; but I have a wider one (+/- 10kHz) on order to see if that will still work okay in the presence of the Drake 2B's 405 kHz local oscillator (which is present on a different grid of the same tube from which the 455kHz output is obtained).

Stu